Cosmic-Ray proton spectrum below 100 TeV in the local region

The propagation of cosmic-ray protons in the Galaxy is discussed under the framework of a three dimensional convection-diffusion model. Starting with the assumption of a uniform and continuous distribution of cosmic-ray sources injecting CRs continuously in the Galaxy and by invoking a supernova explosion at various distances from the Earth, it is found that only those sources located within a distance of ~ 1.5 kpc can produce appreciable temporal fluctuations in the CR proton flux observed at the Earth. So, the construction of the local CR proton spectrum is discussed by seperating the contributions of the distant sources from that of the nearby sources. The contribution from the distant sources is treated in the framework of a continuous source distribution model both in space as well as time, but that of the nearby sources in a discrete space-time source model. The study predicts the presence of at least one old nearby source with a characteristic age of ~ 10^5 yrs located at a distance of ~ 0.1 kpc to explain the observed proton flux below ~ 100 GeV.Comment: 8 pages, 6 figures, 1 table, uses mn2e.cls, minor text corrections, accepted for publication in MNRA

Nearby supernova remnants and the cosmic-ray spectral hardening at high energies

Recent measurements of cosmic-ray spectra of several individual nuclear species by the CREAM, TRACER, and ATIC experiments indicate a change in the spectral index of the power laws at TeV energies. Possible explanations among others include non linear diffusive shock acceleration of cosmic-rays, different cosmic-ray propagation properties at higher and lower energies in the Galaxy and the presence of nearby sources. In this paper, we show that if supernova remnants are the main sources of cosmic rays in our Galaxy, the effect of the nearby remnants can be responsible for the observed spectral changes. Using a rigidity dependent escape of cosmic-rays from the supernova remnants, we explain the apparent observed property that the hardening of the helium spectrum occurs at relatively lower energies as compared to the protons and also that the spectral hardening does not persist beyond $\sim (20-30)$ TeV energies.Comment: 6 pages, MNRAS accepted, minor text correction

GeV-TeV cosmic-ray spectral anomaly as due to re-acceleration by weak shocks in the Galaxy

Recent cosmic-ray measurements have found an anomaly in the cosmic-ray energy spectrum at GeV-TeV energies. Although the origin of the anomaly is not clearly understood, suggested explanations include effect of cosmic-ray source spectrum, propagation effects, and the effect of nearby sources. In this paper, we propose that the spectral anomaly might be an effect of re-acceleration of cosmic rays by weak shocks in the Galaxy. After acceleration by strong supernova remnant shock waves, cosmic rays undergo diffusive propagation through the Galaxy. During the propagation, cosmic rays may again encounter expanding supernova remnant shock waves, and get re-accelerated. As the probability of encountering old supernova remnants is expected to be larger than the younger ones due to their bigger sizes, re-acceleration is expected to be mainly due to weaker shocks. Since weaker shocks generate a softer particle spectrum, the resulting re-accelerated component will have a spectrum steeper than the initial cosmic-ray source spectrum produced by strong shocks. For a reasonable set of model parameters, it is shown that such re-accelerated component can dominate the GeV energy region while the non-reaccelerated component dominates at higher energies, explaining the observed GeV-TeV spectral anomaly.Comment: 12 pages, A&A accepte

On the contribution of nearby sources to the observed cosmic-ray nuclei

The presence of nearby discrete cosmic-ray (CR) sources can lead to many interesting effects on the observed properties of CRs. In this paper, we study about the possible effects on the CR primary and secondary spectra and also the subsequent effects on the CR secondary-to-primary ratios. For the study, we assume that CRs undergo diffusive propagation in the Galaxy and we neglect the effect of convection, energy losses and reacceleration. In our model, we assume that there exists a uniform and continuous distribution of CR sources in the Galaxy generating a stationary CR background at the Earth. In addition, we also consider the existence of some nearby sources which inject CRs in a discrete space-time model. Assuming a constant CR source power throughout the Galaxy, our study has found that the presence of nearby supernova remnants (SNRs) produces noticeable variations in the primary fluxes mainly above ~ 100 GeV/n, if CRs are assumed to be released instantaneously after the supernova explosion. The variation reaches a value of ~ 45% at around 10^5 GeV/n. Respect to earlier studies, the variation in the case of the secondaries is found to be almost negligible. We also discuss about the possible effects of the different particle release times from the SNRs. For the particle release time of ~ 10^5 yr, predicted by the diffusive shock acceleration theories in SNRs, we have found that the presence of the nearby SNRs hardly produces any significant effects on the CRs at the Earth.Comment: 12 pages, 8 figures, 2 tables, accepted for publication in MNRA

Effect of nearby supernova remnants on local Cosmic-Rays

We study in detail the effect of different particle release times from sources on the cosmic-ray (CR) spectrum below $10^{15}eV$ in the Galaxy. We discuss different possible forms of particle injection such as burst-like injection, continuous injection for a finite time, injection from a stationary source and energy dependent injection. When applied to the nearby known supernova remnants, we find that the observed CR anisotropy data favour the burst-like particle injection model for the CR diffusion coefficient $D(E)\propto E^a$ with $a=0.3-0.6$ in the local region. In the study we have also found that the contribution of the sources G114.3+0.3 and Monogem dominate if the observed anisotropy is a result of the effect of the nearby sources. Further study shows that we should not neglect the contribution of the undetected old sources to the local CR anisotropy.Comment: 7 pages, 3 figures, MNRAS accepted, minor text correction

Effect of topology on the collapse transition and the instantaneous shape of a model heteropolymer

The effect of topology on the collapse transition and instantaneous shape of an energy polydisperse polymer (a model heteropolymer) is studied by means of computer simulations. In particular, we consider three different chain topology, namely, linear (L), ring (R) and trefoil knot (T). The heteropolymer is modeled by assigning each monomer an interaction parameter, $\varepsilon_i$, drawn randomly from a Gaussian distribution. Through chain size scaling the transition temperature, $\theta$, is located and compared among the chains of different topogies. The influence of topology is reflected in the value of $\theta$ and observed that $\theta(\text{L}) > \theta(\text{R}) > \theta(\text{T})$ in a similar fashion to that of the homopolymer counterpart. Also studied chain size distributions, and the shape changes across the transition temperature characterised through shape parameters based on the eigenvalues of the gyration tensor. It is observed that, for the model heteropolymer, in addition to chain topology the $\theta$-temperature also depends on energy polydispersity.Comment: 10 pages, 9 figure